Magnetic Contacts

A magnetic contact has two parts; the contact with one or more reed switches and the magnet. In its neutral position, the reed switch remains closed under the force of the magnetic field. Opening the monitored object increases the distance between the reed switch and the magnet. This reduces the influence of the magnetic field on the reed switch until it opens and activates an alarm.

Only the highest quality and performance switches are used in all Alarmtech contacts, Features such as the excellent contact function down to 1 microampere, stick-free resistant contact, virtually immunity to transients and shock and with an extremely long operating life makes Alarmtech contacts ideal for the most demanding applications. The most important part of a reed switch is the contact material. The most common material used in switches within the security industry is Rhodium for electro-plating the contacts. Rhodium is a hard, precious metal that is used as the plating material in the electrolytic process. Microscopic particles of the electrolyte remain on the contact surface that can cause spontaneous short breaks in the contact function at low current thus causing false alarms. To ensure safety loop currents higher than 10 mA are recommended in those contacts. In all Alarmtech switches a further precious metal, Ruthenium, is used. It is much harder than Rhodium and must be sputtered onto the contact. Sputtering is a plating process performed in a vacuum chamber and ensures that no impurities remain on the contact that can affect its conductivity. These switches maintain excellent performance at very low loop-currents, which are commonly used in modern microprocessor control panels. Other benefits gained from using this material are longer operational life and the ability to switch high current loads due to its hardness.

The two materials used in Alarmtech magnets are ALNICO 5 and NEODYMIUM. When  mounted parallel to each other a long magnet is usually necessary for wide separation effectiveness between magnet and contact. For those applications, Alarmtech uses ALNICO 5 magnet material as it has a relatively good energy content per unit volume as well as temperature stability. The energy content per unit volume defines the size of a magnet for a given distance between contact and magnet. For wide separation effectiveness with short magnets when contact and magnet are mounted end-to-end the material Neodymium is used; a high technology, rare earth alloy made of Neodymium, Boron and Iron powders sintered together. 
The magnetic energy content per volume of these magnets is one of the highest possible that yields very small and short but extremely effective magnets. Their resistance to demagnetisation is very high and all of these attributes make it a perfect material for end-to-end mounting. In order to design an effective magnet, the relation between diameter and length and the characteristics of the material used must be carefully considered to ensure that the magnetic field is not drained after a period of time. 

There are two distances to be considered; the closing distance when the magnet is approaching the contact and the opening distance when the magnet is removed from the contact. The opening distance is about 10% longer than the closing distance because the contact requires a stronger field to close the contact.

Alarmtech contacts are designed for mounting on steel doors, safes, filing cabinets, gates and roll-up garage doors. There are many specially-designed accessories for these kind of applications. Mounting on steel does not drain the magnet, however, the magnetic field is diverted from the contact that reduces its magnetic field and, as a result,  a reduced gap is required.

High security contacts have an extra reed switch inside to protect the contact from sabotage with an external magnet. This extra reed switch can be adjusted in two ways. A first solution is to have a small magnet inside the contact with a magnetic field directed opposite to the normal magnet and with enough field strength to keep the extra reed closed but not the normal reed. The extra reed switch is biased. A second solution is to have the extra reed with a reduced sensitivity connected across the loop. This model is suitable for balanced loops. For high security contacts two distances must be determined. The first distance is the closing distance when the contact approaches the magnet and the second is when the sabotage reed opens when the magnet is very close to the contact. To ensure the highest security always place the magnet as close as possible to this second distance.